By J. R. Watson
Since so many projects in Epoxyworks incorporate plywood, we felt it might
be valuable to discuss briefly the types of plywood and some construction methods
best suited to it. It's easy to understand why people like plywood and choose
it for so many projects: it is readily available, comes in convenient sheets
(typically 4'x 8'), is pretty light for its stiffness and strength (1/8" plywood
weighs about 11 lb per 32 sq ft panel), and is a bargain when compared to the
price of many composite panels. However, plywood also has its weak points. There
are limits regarding shape development because plywood can be compounded (bent
in two directions at once) very little. In addition, plywood contains end grain
on all its edges, which makes it susceptible to moisture migration and damage.
So while we've had good success in a diversity of projects using plywood, it's
important to consider these guidelines.
Understand plywood grades
All plywood is graded based on the quality of the veneers and the size and number
of permissible voids and patches. Grades A-C are the most common. To be suitable
for marine construction, panels must be made with waterproof glue and marked
as exterior plywood (EXT) or marine plywood.
As a general rule for boat projects, use marine-grade plywood
because the wood used to make it is of higher quality with smaller voids and
fewer patches. Inner veneers are also of higher quality, grade B or better.
Marine plywood is therefore more stable, has less potential for checking, and
the overall strength and stiffness are better than exterior plywood. When you
consider the overall cost of a project, the price for marine grade plywood is
well worth it. While AC exterior plywood could be used on "low cost" boats,
filling all the knots and voids may cost more in time and materials than if
you had bought better grade plywood. Aircraft grade plywood is also very high
quality and might be appropriate in specialized applications, such as a cored
panel wing mast. Some builders have used door skins (3mm thick) when making
a laminate of multiple layers, but you need to make sure they are glued with
exterior grade glue and are of adequate quality. You should not use plywood
underlayment for boat building because of its poor quality.
U.S. marine grade plywood must meet U.S.
Products Standard PS 1-74 (applies only to softwood plywood).
Species: Douglas fir No 1 or western larch. Glue:
waterproof. Face and back veneers A-A, A-B, B-B smooth cut and free of knots,
pitch pockets and open splits and other open defects. Inner plies B or better.
Repairs: no more than 9 repairs allowed on A faces in a 4x8 sheet.
Marine grade for tropical hardwoods must
meet British Standard Specification BS 1088.
Species: untreated tropical hardwood veneers
with suitable level of resistance to fungal attack. Glue: waterproof glue.
Face veneers free of knots and other than solid pin knots no more than 6 in
any 1 square foot area. Core veneers shall be the same as those for face veneers.
No gaps in faces. In core, not more than one gap no wider than .5mm. Face
veneers if three-ply construction shall not be thinner than " of the total
Aircraft grade plywood meets all specifications
of MIL-P-6070, which calls for shear testing after immersion in boiling water
for three hours. It is made of imported African mahogany or American birch
veneer laminated in a hot press to a hardwood core of poplar or basswood with
Check the plies
Plywood typically comes in an odd number of plies in order to achieve balanced
construction. The number of veneers on each side of the core layer must be equal
in number (and thickness) or the panel may warp. The plies should be parallel
to one another with alternating layers set at 90° or warping can occur. When
you buy plywood, lay the sheet down on the ground. If it's curled or twisted
and doesn't lie flat, don't buy it. Select plywood where the total thickness
of the two outer plies equals the thickness of the core. Avoid 'cigar box' stock
where a thick core is sheathed with thin face veneers.
Most plywood under 1/4" (6mm) is three ply construction. One quarter inch plywood
and thicker is available in 5 or more plies. More plies are better because the
panel is more dimensionally stable, less likely to warp and stiffer than three-ply.
Know the wood species
Most marine grade plywood is made from tropical hardwoods, like African mahogany,
sapele, utile, and okoume. Some wood species are more durable than others. Coating
surfaces (especially exposed end grain) with epoxy greatly protects less durable
woods from the harmful effects of moisture. While okoume is considered a non-durable
species, we have had good long-term results with it. Much of this is the result
of thorough coating with WEST SYSTEM® epoxy and careful maintenance of the finished
boat, including ventilation and prompt repairs.
The biggest challenge with Douglas fir plywood is the propensity of its outer
(face) veneer to check, that is, split along the grain. This is especially true
when the outer surface forms a curve perpendicular to the face grain. Covering
fir plywood with 6 oz fiberglass fabric is mandatory to contain the checking.
Limitations of pressure-treated plywood
Many people ask about the value of pressure treated plywood. Often, pressure
treated plywood is heavily warped. This is primarily the result of the high
temperature and rapid moisture take up during processing. Some plywood delaminates
during the process. In addition, some pressure treating methods incorporate
water repellent materials. These materials can easily jeopardize bonding so
you won't be able to glue or paint the plywood successfully. Generally speaking,
pressure treated plywood may be acceptable for house projects but not for boats.
Select the right plywood for the project
With the many types of plywood available, it is important to choose and combine
the types and thicknesses of plywood best suited to your project. For the kind
of projects typically described in Epoxyworks, strength, durability and weight
are primary considerations. So we usually choose somewhat specialized plywood
to meet the particular requirements of the project. For example, on a wing mast
I built twenty years ago, I was after light weight and flexibility. So, I selected
plywood that would allow me to bend the laminate to a tight radius foil section:
aircraft grade 1/16" birch on the mast's exterior, and 1/64" (3 ply) on the
interior, with a honeycomb core. On a current project, I've chosen a 9 mm teak-faced
plywood to replace the live well and wet locker hatches on my Mako Angler. I
wanted the teak finish and flat, hinged panels. So I chose a plywood with a
teak face, stiffness and dimensional stability. The point is to select carefully
the best plywood for your project.
Building your own plywood
Some folks opt to build their own plywood by laying up multiple layers of sliced
or peeled veneer. You need a suitable flat surface and a method of applying
pressure until the adhesive that joins the layers has cured. Building your own
plywood rarely saves money and the panels are often heavy. However, you can
tailor your choice of a face veneer and it is possible to make some really beautiful
panels. You also can make the panels whatever size you want. When you laminate
a hull, as in cold-molded construction, you are actually making plywood to a
specific shape, that of a boat hull (see Laminated construction gives lasting value).
Joining plywood panels
You can join plywood panels in a number of ways if you need sizes greater than
standard ones. Scarf joints are the best approach for strength and weight. (If
you order 16' long panels, offered by some plywood suppliers, the panels are
simply scarfed together, but you will pay a premium both for the material and
the shipping.) Gougeon Brothers, Inc. does offer a custom attachment (875 Scarffer)
that fits onto a circular saw. This tool makes scarfing plywood panels of 3/8"
thickness or less quick and accurate. You can also fashion scarfs with a plane
and/or grinder. The rule is to create a bevel 8 times the thickness of the plywood.
Panels can also be joined with butt blocks, but this has the potential for exposing
end grain at the butt. We like to dish out the butted area similar to a scarf
and bond in fiberglass cloth.
Construction methods with plywood
Plywood is ideal for several construction methods. The most common is "stitch
and glue" construction where panels are cut to a designed shape and temporarily
joined with wire or plastic ties. Then you apply epoxy fillets and fiberglass tape to permanently join the structure in a rigid shape. (See Building prams) Plywood has also been used successfully to build lightweight
lapstrake boats. Tom Hill described this in Ultra Light Boatbuilding (available
through International Marine Publications, phone 207-236-4837).
Probably the most extreme shapes can be gotten via "tortured," also known as
developed plywood construction, where the plywood is actually compounded. This
construction method is limited to thin plywood and hulls with modest shapes,
most typically multihull craft. Designing your own plywood boat used to be very
challenging because if you introduced excessive compounding, you could have
trouble laying out the plywood to the shape. With stitch and glue construction,
it is extremely difficult to predict panel shapes. Now, however, some excellent
software is available so you can get accurate offsets for the true shapes of
panels. Two that we have used are Plyboats, 714-840-7284, www.plyboats.com and
New Wave Systems, Inc., 401-423-1852, www.newavesys.com.
Epoxyworks 18 / Fall 2001
Copyright © 2002, Gougeon Brothers, Inc. All rights reserved.
Last Modified on 10/28/02.
Reproduction in any form, in whole or in part, is expressly forbidden without the consent of the publisher. EPOXYWORKS, Gougeon Brothers, WEST SYSTEM, Episize, Scarffer and Microlight as used throughout this publication, are trademarks of Gougeon Brothers, Inc., Bay City, Michigan, USA.